Electronic Properties of Single Silver Nanoparticles Are Studied

Metallic nanoparticles display potential for such applications as enhancing the fluorescence of biological molecules, but for all the interest, significant questions regarding their properties remain. The electronic properties that affect the interaction of the particles with their environment, for example, remain relatively uncharacterized.

By measuring the optical transmission of laser pulses tuned to the surface plasmon resonance wavelength of silver metallic nanoparticles, researchers have characterized individual particles. Adding time-resolved spectroscopy to the technique further enables the examination of electron-lattice coupling.
Natalia Del Fatti and colleagues at the Centre de Physique Moléculaire Optique et Hertzienne at Université Bordeaux 1 in Talence, France, have developed an optical method that can measure the electronic properties of a single metallic nanoparticle. They demonstrated the technique using batches of two sizes of silver particles that they dispersed on glass substrates at areal densities of less than one particle per square micron.

A homebuilt Ti:sapphire laser, doubled through a BBO crystal, provided 100-fs pulses tunable from 400 to 450 nm, the region of the anticipated surface plasmon resonance of the particles. A piezoelectric transducer sinusoidally varied the position of a single nanoparticle through the focal spot of the laser. A lock-in amplifier detected the resultant modulation in the transmitted optical power, providing a measurement of the absorption of a single particle. By tuning the excitation wavelength, the investigators measured the peak absorption wavelength and the absorption linewidth of the structures. The two sizes of nanoparticles exhibited different absorption characteristics, enabling them to identify the particle sizes as 21 and 30 nm.

They then used 850- and 425-nm pump and probe pulses from the laser in a femtosecond time-resolved spectroscopy method. Electrons excited by the infrared pulse take some time to relax to their initial state, and the transmission of the nanoparticle varies as a function of the excitation state. By varying the time delay between the pump and probe pulses, they could create a picture of the electron-lattice interaction dynamics.

To connect the transmission measurements to electron kinetics, the excitation temperature of the electrons must be known. That is difficult in ensemble measurements. Because the individual nanoparticles had been optically characterized in the first step of the experiments, however, the researchers could more accurately determine the absorbed energy and confirm the predictions of a model of the rate at which electrons transfer energy to the lattice vibrations.

With the initial validation of the technique complete, Del Fatti said that the scientists will perform single- particle optical characterization and in situ time- and frequency-resolved spectroscopy on nonspherical nanoscale objects. Fascinated by the subtleties of this method, she said that it not only allows researchers to visualize objects that are much smaller than the optical wavelength, but also enables them to “listen” to what the particles have to say about their size, orientation, anisotropy, local environment and dynamic response.

The emission of light or other electromagnetic radiation of longer wavelengths by a substance as a result of the absorption of some other radiation of shorter wavelengths, provided the emission continues only as long as the stimulus producing it is maintained. In other words, fluorescence is the luminescence that persists for less than about 10-8 s after excitation.